Fix avatar alt-text running into other elements on image load failure. (#38000)

app/javascript/mastodon/components/__tests__/__snapshots__/avatar_overlay-test.jsx.snap

@@ -26,6 +26,7 @@ exports[`<AvatarOverlay > renders a overlay avatar 1`] = `
     >
       <img
         alt="alice"
+        onError={[Function]}
         src="/static/alice.jpg"
       />
     </div>
@@ -44,6 +45,7 @@ exports[`<AvatarOverlay > renders a overlay avatar 1`] = `
     >
       <img
         alt="eve@blackhat.lair"
+        onError={[Function]}
         src="/static/eve.jpg"
       />
     </div>